The present invention relates to material testing. More particularly, the invention is directed to a method and apparatus for monitoring the planar flatness of web material.
Paper, plastic film and other materials commonly used in sheet and strip form are often initially produced as a continuous material length that is referred to as a xe2x80x9cwebxe2x80x9d. Upon reaching the end of the continuous production line, the web product is reeled about a core into a cylindrical xe2x80x9crollxe2x80x9d. Along its production process line, a web will typically will be subjected to several processing steps before a final product is produced such as coating and/or saturating. In these processing steps, the web will be drawn over a number of roller type devices where it is stretched under tension while subjected to moisture and temperature changes, or both, to craft certain web characteristics. However, these temperature, moisture and tension variations may also cause the web to stretch or shrink unevenly across the crossweb direction and in the machine direction of the web. When the web returns to its steady-state condition, the stretched or shrunk areas will often cause warp and sag in the web flatness: conditions that are often characterized as bagginess. These changes in the condition of the web can cause problems in later web handling operations, including winding and converting. For example, it is more difficult to properly slit a creased or wrinkled web into strips than to slit a web without bagginess problems.
Web bagginess is particularly a problem in the crossweb or transverse direction, which is the direction that is transverse to the longitudinal or machine direction of the web as it is running through a manufacturing process. Contact methods of monitoring the bagginess of a web typically use a device that physically contacts the surface of the web. One method of measuring web bagginess is to stop the web handling process and use a yard stick or tape measure to measure the approximate width of the web at a particular location. This width measurement is compared to the expected width of the web. If the actual measured width is wider than the expected width, it is likely that the web has stretched and appropriate process adjustments can be made to compensate for web bagginess.
Another example of a contact method of monitoring web bagginess of a moving web is described in U.S. Pat. No. 5,317,913 (Feistkom et al.). Feistkorn et al., describes a web sensing roller that contacts the surface of the web, where a carrier of the web sensing roller is guided within a housing and movable in a direction perpendicular to the plane of the web. The web sensing roller presses against the surface of the web as the web moves past the web sensing roller. The apparatus measures the distance that the web sensing roller moves form a predetermined reference position, which corresponds to the bagginess in the web.
For some web converters and conversion processes supply web bagginess causes no major difficulty. For others, the condition generates expensive or unacceptable difficulty. Tests for or measurement of web warp or bagginess that are carried out on the web production line, therefore, may be inappropriate for all of the web product that is consolidated onto a single production roll if that product is to be distributed to several converters for different product purposes.
Another prior art test for web bagginess has been the highly subjective practice of xe2x80x9csoundingxe2x80x9d a rolled length of the web with a bat, club or stick. The bat is struck against the roll ends with an educated swing. The resulting sound and feel of the impact is evaluated against the tester""s experience.
It is an objective of the present invention, therefore, to teach a bagginess test procedure and corresponding apparatus that may be used selectively and quickly.
Another object of the invention is to provide a bagginess test apparatus that is small, light and highly portable.
Also, an object of the invention is provision of a bagginess test procedure that is direct and easily carried out by persons having minimum training.
A further object of the present invention is a bagginess monitoring procedure and apparatus that may be selectively applied to meet diverse needs and specifications for particular converters and products.
A still further object of the invention is to provide web manufacturers and processors with a tool for objectively measuring the amount of bagginess in a sheet of paper.
As a corollary to the foregoing objective, it is an object of the present invention to provide web manufacturing and processing personnel an easily and conveniently employed tool that may be used to avoid shipment of web products that do not conform to the intended customer""s needs.
These and other objects of the invention to be hereinafter described are obtained by use of the method and apparatus to carry out the corresponding monitoring procedure. In a preferred embodiment of the invention, a pair of structural straight edges fabricated from xc2xe inch square aluminum tubing, for example, are erected in substantially horizontal parallelism along an approximately three-foot separation space to establish a reference plane. Length of the straight edges is somewhat arbitrary beyond a length that is at least as wide as a section of the web under examination. However, a length of about three to four feet is preferred for convenience of manual manipulation.
A physical comparison line in the form of a tensioned wire or string is secured between the straight edges and parallel therewith. However, the comparison line position is located in a plane that is below the reference plane about xe2x85x9 inch to about xc2xd in and parallel therewith.
A machine direction length of the web, about five feet for example, is drawn across the straight edges and aligned in the reference plane with sufficient tension to give the web substantially contiguous contact along both straight edges. So positioned, any planar warpage or bagginess in the sheet will manifest in the form of sheet material sag below the reference plane. Location of the comparison line establishes a tolerable sag limit. Sag in the material under evaluation that is sufficient to contact the comparison line may be judged as meeting or exceeding a predetermined limit and be considered unsuitable for the purpose intended.
Positionment of the comparison line with respect to the reference plane is set empirically as an experience determined function of the web grade, the intended conversion process, specifications acceptable to the converter and other such considerations.
An especially usefill configuration of the invention includes a pair of scissor frames. Opposite ends of each straight edge are secured to respective upper ends of four support legs. The two legs respective to each of the two scissor frames are crossed in an xe2x80x9cxxe2x80x9d configuration and secured together at the x intersection. A compression gusset is secured to the legs of each frame above the leg intersection and across the upper crotch of the intersection. At or slightly below the upper ends of the legs respective to each frame, a guy wire or cable is attached to and between the leg ends and prestressed in tension. These wires are preloaded as by a spring or turnbuckle. This arrangement produces great rigidity for the reference frame straight edges. Simultaneously, the entire structure may be extremely light and conveniently portable.